Experiments have been performed on microglia and astrocytes in the central nervous system cultured from cerebral cortices of rat and hamsters. We have previously shown that activated microglial rat cells produce the superoxide radical anion, a reactive oxygen species (ROS). ROS include also the hydroxyl radical, hydrogen peroxide, and nitric oxide. Activated rat microglia also produce nitric oxide. However, the release of nitric oxide does not significantly increase until after about 10 hours of stimulation, whereas the release of the superoxide radical anion is completed within 2 to 3 hours of stimulation. Also, hamster microglia produce more than ten times the amount of the superoxide radical anion produced by rat microglia. In all the conditions we have tested, there is no evidence that the hamster microglia releases the nitric oxide. In another set of experiments, we have shown that N- acetyl-cysteine increases the survival of tyrosine hydroxylase-positive rat mesencephalic neurons grown in tissue culture. In addition, there are more surviving rat mesencephalic neurons at an oxygen pressure of 38 torr than at the commonly used oxygen pressure of 150 torr.